A diffuse reflectance method and apparatus are used to determine thickness of an infrared translucent layer on a metal substrate.
In a past, collimated beams, coming from an interferometer, were used to produce an interferogram of a relatively thick silicon layer that had been epitaxially coated onto a silicon wafer. A single angle of incidence was made with the silicon layer, by collimated interferometer beams. Reflections of the collimated beams were produced. Again, collimated interferometer beams were used in the past to form an interferogram. The interferogram was used to determine the thickness of the relatively thick silicon layer.
The present diffuse reflectance method and apparatus provide for an accurate measurement of thickness of a relatively thin infrared translucent layer on a metal substrate. An example of such a relatively thin translucent layer on a metal substrate is a thin beryllium oxide region that is formed into a beryllium metal part.
The present method uses diffuse reflectance to measure thickness of the relatively thin beryllium oxide region. Parallel infrared interferometer beams are emitted from an interferometer. A concave mirror is used to converge the parallel beams into converging infrared interferometer beams. Converging interferometer beams are sent onto both the beryllium oxide region and underlying beryllium substrate. Diverging infrared interferometer rays are diffusely reflected from the beryllium oxide region and beryllium substrate, after reflection of the converging infrared interferometer beams from the beryllium oxide region and from the beryllium substrate.
The diverging interferometer rays are collimated and analyzed by means of Fourier transform infrared spectroscopy.
A concave mirror is used in the present apparatus to collimate, that is make parallel, the diverging interferometer beams.
Different angles of incidence are made between the converging interferometer beams and a line perpendicular to the surface of the beryllium oxide region.
A movable mirror of the inteferometer is scanned and the overall intensity of interfereing diffusely reflected rays, coming from the beryllium oxide region and beryllium substrate, is detected. An inteferogram is produced by recording the intensity versus the increment amount of scan distance. Sidebursts occur in the interferogram due to reflections above and below the oxide region. An amount of displacement of a first sideburst, in the interferogram, from a centerburst, in the interferogram, is measured. The amount of displacement is indicative of the thickness of the beryllium oxide region.
The present diffuse reflection method and apparatus were used to measure the thickness of a beryllium oxide region that had been formed in a beryllium substrate. Beryllium oxide thicknesses ranging from 0.67 microns to 4 microns were measured.
A beryllium oxide region is formed in a beryllium substrate by oxiding the beryllium substrate. The beryllium oxide region could be a beryllium oxide region that was formed by oxidizing a beryllium part.
It is noted that when a beryllium substrate is oxidized, there is a less uniform interface region than the interface region that occurs when a silicon layer is epitaxially placed on a silicon wafer.
A prior art software program, that had been used in the prior art measurement of a thickness of a relatively thick epitaxial silicon layer coated onto a silicon wafer, was modified. The software was used to measure thicknesses of the silicon layers that had thicknesses that ranged from 25 microns to 150 microns. The prior art software program operated by subtracting an interferogram of the epitaxial layer under examination from an interferogram of a reference epitaxial layer of known thickness. The resultant subtracted interferogram was searched by the software until a first major sideburst was found. At this point, the program calculates the thickness of the silicon epitaxial layer, using the distance of the sideburst from a centerburst of the subtracted interferometer, and the refractive index of the silicon epitaxial material.
Again, the prior art software program operated by subtracting an interferogram of the silicon layer under examination from an interferogram of a reference silicon layer of known thickness. The resultant subtracted interferogram was searched by the software until a first major sideburst was found. At this point, the program calculates the thickness of the silicon layer, using the distance of the sideburst from a centerburst of the subtracted interferometer, and the refractive index of the epitaxial silicon.
The prior art software program was modified in order to be used with the new method and apparatus. The modified software program can be used to measure the thickness of a relatively thin beryllium oxide region in a beryllium substrate. Such an oxide region might have a thickness from between 0.67 microns to 4 microns. A refractive index value of 1.8 was selected for a beryllium oxide region, in the modified software program. This value is used with the modified software program.
As part of its broad scope, a producibility program supported applications of new advancements from a wide range of technical disciplines to improvements of manufacturing and testing techniques for instruments. In this regard, steps were taken to prove that a nondestructive thickness measurement of beryllium oxide regions, in anodized beryllium components, was possible using diffuse reflectance Fourier transform infrared spectroscopy. Further steps were taken to demonstrate that the method could be readied for production use. By first demonstrating the interferometric principle with spectral patterns obtained from anodization regions of varying thicknesses, the modified prior art software program was incorporated into the disclosed method.
The modified software program enabled automated, nondestructive beryllium oxide region measurement by operators on a production line. The software program operated to the satisfaction of production and design engineers.
The disclosed diffuse reflection Fourier transform infrared spectroscopic method and apparatus, for measuring thicknesses of beryllium oxide regions, developed under the producibility program, can provide useful processing information about anodization region thickness and region uniformity, and to determine changes in the region""s chemical composition. An implementation plan was developed by production engineers to determine how the disclosed method and apparatus, and the information that it generates, could be used.
When a group of beryllium components are manufactured, several production samples are routinely destroyed by an acid-etch technique wherein one obtains the thickness of the anodization, that is oxide, region, by etching away a small area of the oxide region and measuring the resultant hole with a form tally instrument. Aside from the destruction of useful hardware, there is some question regarding the accuracy of the acid-etch techniques. This issue warranted the use of an alternate thickness measurement method.
The disclosed diffuse reflectance method and apparatus will preserve hardware and improve the accuracy and efficiency for determining oxide region thicknesses.
A method for producing an interferogram of an infrared translucent layer that is on a reflective substrate, comprising generating parallel infrared interferometer beams by means of an infrared interferometer, converging the parallel infrared interferometer beams into converging infrared interferometer beams, sending the converging infrared interferometer beams onto the infrared translucent layer to produce diffusely reflected infrared interferometer rays from above and below the infrared translucent layer, and making the diffusely reflected infrared interferometer rays into parallel reflected infrared interferometer rays.